The present invention is directed generally to the signal processing arts and more particularly to a novel and improved signal processing method and apparatus for use with a radar transceiver and associated circuitry for determining the velocity of a vehicle.
The prior art has proposed a number of radar-based systems for determining the velocity of a vehicle. Such systems are generally directed to the problem of monitoring the horizontal velocity of a vehicle travelling along a ground surface. Such radar speed monitors may be suitable for use from a fixed location in monitoring the speed of passing vehicles or alternatively, mounted upon the vehicle itself for monitoring of the speed thereof by an operator or driver of the vehicle.
The latter type of velocity monitor is particularly useful with respect to off-road vehicles, or farm implements, such as tractors or the like. In such vehicles, axle-mountd or other traditional velocity monitoring devices may become unreliable and/or inaccurate due to frequently encountered wheel slippage conditions in off road operation. Also, uneven ground conditions may add substantial vertical velocity components which tend to interfere with accurate speed measurement. Additionally, compaction of the wheels during operation, or changing of wheel sizes due to wear, or replacement thereof over a period of time will result in a change in the effective diameter of the wheel, whereby conventional wheel-shaft or drive-train based rotation responsive speed monitoring apparatus may prove inaccurate in operation. Radar-based velocity monitoring apparatus has been heretofore proposed for overcoming these problems.
One such radar-based velocity monitoring system for a tractor is shown for example in Fathauer et al U.S. Pat. No. 3,895,384. While this monitoring apparatus has found widespread acceptance, there is room for further improvement.
In particular, such a radar system generally uses a radar frequency transceiver comprising a radar oscillator, an antenna, local oscillator and mixer, to transmit and receive radar signals. The mixer operates to mix or multiply the received radar signal with the local oscillator signal so as to provide at least one difference component signal in an intermediate frequency (IF) range, preferably in the kilohertz range, suitable for further processing by conventional electronic circuit components. In this regard, the operation is analogous to that of conventional radio transmission involving a carrier wave similar to the transmitted radar signal and a signal component carried upon the carrier wave which is analogous generally to the Doppler shift or frequency variation in the received radar signal. Hence, only this difference is of interest in determining the corresponding horizontal velocity component of the vehicle.
As in all such radar and/or radio apparatus, various harmonic signal components, spurious signals and/or noise signals in general can interfere with proper operation of the circuit and ultimately with the correct determination of the speed of the vehicle. Accordingly, filtering is often utilized to limit the following processing circuits to substantially only the frequencies of interest, that is, the range of frequency variations expected in response to the expected range of vehicle velocities. In this regard, such off-road vehicles may be operated in a range of speeds from on the order of fractional miles per hour to on the order of tens of miles per hour.
Additionally, it is often desirable to shut off the radar antenna portion of the system when the vehicle is not in actual use in the field, or under other conditions. For example, when the vehicle is not in use, it may be considered desirable to minimize production of microwave radiation from the radar antenna. This may be true in the event of replacement, repair or repositioning of the antenna or other parts of the radar apparatus or of components of the vehicle which are located near the antenna.
Additionally, it is often desirable to cut off the velocity monitoring function of such a system at relatively low vehicle speeds or when the vehicle is standing still. This avoids possible false response of the system to spurious horizontal velocity signals which may be generated when standing still, for example, by other moving objects within the range of the radar antenna. It will be understood that when the vehicle is in motion, the present invention is such that other such velocity components will be substantially rejected in arriving at the correct velocity indication. However, with the vehicle standing still, such relative movement in the field of the radar may under some circumstances be indistinguishable from movement of the vehicle. Accordingly, it is desirable to at least prevent a velocity signal so generated from reaching the display components of the system, and moreover from reaching any further control apparatus which may operate a further implement in response to the velocity signal output of the monitor.
In order to substantially limit the response of the processing circuits to the signals of interest corresponding to the expected range of speeds, a bandpass filtering circuit is generally utilized. Moreover, in order to further limit the response of the system and reject a maximum amount of spurious, harmonic or noise signals, it is often the practice to operate this bandpass filter in a band considerably narrower than the expected range of speed variations. This is facilitated by utilizing tracking control which varies the center frequency of the relatively narrow bandpass to generally follow the frequency of interest in the incoming signal. This further requires circuitry to accurately detect and identify this frequency of interest or fundamental frequency of the incoming signal.
Additionally, some control over the bandwidth of the bandpass filter is generally considered desirable in order to continue to accurately track changing incoming velocity signals, over a range of frequencies which may vary with changing velocity of the vehicle. That is, the bandwidth should be broad enough, when required, to follow relatively rapid vehicle acceleration or deceleration and the attendant relatively rapidly changing doppler frequencies resulting therefrom. On the other hand, during periods of relatively stable or constant velocity operation, the bandwidth should preferably be kept relatively narrow to maximize spurious and noise signal rejection. However, conventional prior art analog bandpass filtering, tracking and variable bandwidth or Q control circuits are relatively complex and expensive.
The prior art has proposed replacing such complex and expensive analog circuits with equivalent digital circuits, which perform essentially equivalent bandpass filtering, tracking and Q control functions in a digital form, generally utilizing digital computer or processor components. Briefly, the computer model for a digital bandpass filter including variable tracking and Q control involves selecting a sampling rate at least as great as the highest frequency expected to be encountered in the signal of interest. This computer implementation also involves the computation of a plurality of mathematical transformation functions which becomes relatively complex.
As a result, operating at a sufficiently high rate of speed to accurately sample and filter a sinusoidal signal of a frequency of on the order of even one or two kilohertz requires a surprisingly large amount of computing power. This is true because of the large number of computations which must be performed with respect to each sample and the relatively high sampling rate at which such samples must be processed in order to adequately follow the incoming signal. Accordingly, relatively inexpensive single-chip microcomputer or microprocessor components which are generally available fall far short in terms of computing power of the requirements for such implementation of tracking bandpass filtering. Accordingly, it was heretofore believed that implementation of adequate bandpass filtering, including tracking and Q control in such a system would require relatively complex and expensive analog components, or alternatively, a digital computer system much too complex and expensive to be economically utilized in a typical off road vehicle or farm tractor application.